A rechargeable secondary battery is receiving attention as a power source for electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in HEVs, which are proposed to overcome air pollution problems of conventional gasoline and diesel vehicles.
Such a secondary battery is used by electrically connecting a plurality of battery cells. As such, the battery cells are connected in such a manner that a bus bar, an anode lead and a cathode lead necessary for electrical connection thereof are linked.
However, when anode and cathode leads made of dissimilar metals are welded to the bus bar, an intermetallic compound between dissimilar metals may be formed, and thus brittleness may increase, undesirably increasing the risk of cracking. Furthermore, when changes in temperature and environment continuously occur, the intermetallic compound between dissimilar metals may grow, whereby the welded portion may crack or may corrode with an increase in resistance. To solve such problems, mainly utilized is a method of connecting the dissimilar metal bus bar and the leads made of dissimilar metals using a clad metal formed by pressure welding. Such a welding method using a clad metal is disclosed in Korean Patent Application Publication No. 10-2011-0109779.
However, the conventional welding method using a clad metal requires an additional process for welding between dissimilar metals, undesirably increasing the manufacturing cost and imposing limitations on process convenience. Hence, a method of welding dissimilar metals needs to be improved.